Cyproheptadine (Periactin) hydrochloride, a histamine and serotonin antagonist, was compared with placebo for effectiveness in producing appetite stimulation and weight gain in healthy, underweight, adults. Increase in appetite and weight was significantly greater statistically in those who received cyproheptadine. Drowsiness was a common transient side effect of cyproheptadine. There were no significant alterations in laboratory values following cyproheptadine administration.